All choked up

Jan. 1, 2020
How do you know if reduced engine breathing causes the drivability issue you’re chasing?

It was easy in the old days. If you wanted more power, you simply added more inches, cubic inches, that is. But carmakers today are faced with increasingly stringent standards for emissions and fuel economy. One of the great challenges here in the U.S. is the Corporate Average Fuel Economy (CAFE) standards requiring a corporate average of 35.5 miles per gallon by the 2016 model year, and stretching out to over 54 miles per gallon by 2025.

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To the credit of the engineers and designers, so far the goals have been met. Engines are being downsized, but the technology they contain allows them to produce the same amounts of power their big brothers were just a few short years ago. Computer controls allow engineers to alter intake tracts across the operating range, change the valve timing to maximize efficiency at low and high speed, all in addition to the precise control they already had over fuel mixture and ignition timing.

But even with the high tech advances, a gasoline engine is still a gasoline engine. More simply, it’s an air pump and the more air we can pack into it, the more fuel we can burn, and the more power we can make. If a restriction forms at either end of the engine (intake or exhaust), it won’t be able to bring in all the air it’s capable of, and performance will suffer. And don’t let the word “restriction” limit your thinking. Anything that impacts airflow through the engine can be a “restriction”. There’s the obvious, like a clogged air filter, and the not so obvious, like retarded cam timing.

But first things first. How do you know if reduced breathing causes the drivability issue you’re chasing?

Volumetric Efficiency
Volumetric efficiency is a comparison between the amount of air actually moved through the engine versus what the engine is theoretically capable of moving. In a healthy normally aspirated engine, volumetric efficiency will peak at a little lower than a perfect 100 percent. Expect normal to be around 80 to 90 percent. If there is help packing air into the engine (turbo or supercharger), that normal is going to exceed 100 percent. Look for numbers in the 100 to 120 percent range.

There are a couple of ways you can test the volumetric efficiency of your problem child. You’ll need a scan tool capable of reading and recording Global OBDII, and an area near your shop where you can perform a few wide open throttle (WOT) runs from a rolling start safely (and legally). The first, and easiest, method uses a standard Parameter Identifier (PID) called LOAD or CALC LOAD. Look for it in the datastream and select it as one of the PIDs you want to display and record. In addition, add these PIDs to the list:  RPM (engine speed), STFT (Short Term Fuel Trim-both banks if applicable), and LTFT (Long Term Fuel Trim-both banks if applicable).

Here’s a tip for you. Most scan tools allow you to choose what PIDs you want to display. By selecting only those you need or want to observe, you speed up the refresh rate of the data.

Now you’re ready for your test run. Connect your scan tool and start recording. From a rolling start, drop the hammer and accelerate at WOT until you feel the shift. You can do this in first or second gear, whatever your area will allow safely. Of course, allow the engine to reach normal operating temperature before you floor it!

Now check your recording. I like to graph the data. It makes that peak RPM easier to see. Locate that peak just before the shift and then look at the corresponding value for LOAD. If it is lower than it should be, check fuel trims. If fuel trims are in normal ranges, you’ve probably got a restriction. If, however, fuel trims are correcting for a lean condition (adding fuel, positive numbers), take a hard look at the Mass Airflow sensor (MAF). It is likely under-reporting airflow to the Engine Control Module (ECM).

Be sure to make a few passes to insure the readings you’re getting are correct.

The LOAD PID is a calculated value, not a measured one. That’s why a bad MAF can skew the whole deal and why it is a valid test of the MAF sensor itself. If you want to be more high tech, there are several VE calculators online that allow you to input the values yourself. If you use one, you’ll need to add a few more PIDs to your list: Intake Air Temperature (IAT) and airflow (in grams per second). You’ll also have to enter the engine displacement precisely for the calculator to return an accurate number.

Coming In or Going Out? If you’re VE test results suggest a restriction to airflow, you’ll need to narrow it down to a problem on the intake side of the engine or the exhaust side. One way you can narrow which half to focus on is an “old school” test called the running compression test, using your mechanical compression gauge.

First, do a normal cranking compression test. Be sure the battery is healthy, the fuel system is disabled (to avoid washing the cylinders down), and the throttle is wide open. Record the readings for all cylinders. Next, enable the fuel system and reinstall any spark plugs you’ve removed except for one. Put your compression tester in this hole, leaving the line’s Schrader valve in place.

With the compression tester installed, start the engine and “burp” the gauge a few times using the pressure relief on the tool itself. Then snap the throttle to WOT and record the peak reading on the gauge. Repeat, one cylinder at a time, until you’ve gotten results for all of them.

Normal snap throttle running pressures are roughly 80 percent of cranking compression pressures. If the restriction is preventing air from reaching the combustion chamber, this number will be lower. And if the restriction is preventing the air from getting out through the exhaust, the number will be higher.

If you’ve added a pressure transducer to your lab scope arsenal, you can use it to perform an in-cylinder running pressure test. This method not only show you that there is a restriction, it can also provide the clues you need to identify the cause of the restriction. Problems with valve openings and cam timing are easily seen using the technique. Build up in the exhaust plateau (pressure) can indicate a plugged catalytic converter, even if only one side of a dual cat system has a problem.

Here’s a tip: If you do have a clogged exhaust on only one side of a dual cat system, take a look at fuel trims. One bank will typically show a lean correction, while the other side will show a rich correction. Why? The MAF is measuring total airflow and the ECM thinks each bank is getting an equal share, adjusting fuel accordingly. In reality, one side is getting more than the other. The oxygen sensor on the side getting more is reporting a lean condition and the ECM adds fuel. The sensor on the side getting less is reporting a rich mix and the ECM takes fuel away. The side with the negative trim is the side with the restricted exhaust.

OK, I know, what if I can’t get to the holes on the backside of the engine?

Additional Testing Tips
Some engine designs won’t allow you to perform a running compression test on every cylinder. It’s hard to run the engine with the plenum on the bench. If that is the case, then it’s time for a different approach.  You’ll have to break the engine in half (not literally!), and do a little old-fashioned troubleshooting. Let’s start with the easiest first.

Take a good look at the intake tract. Look at the condition of the air filter. Is it an aftermarket part? Perhaps it’s been over-oiled? Any critters living in the air box or its inlet? You can always try rerunning the VE test with the air filter removed to see if VE returns to normal or power is restored. I’ve had a few instances where the replacement filter was just too restrictive, even though it was brand new. And don’t forget to take a look at the inlet screen on the MAF sensor. It’s amazing what can get sucked up into the intake tract.

Next easiest step is to check for excessive exhaust backpressure using a manifold vacuum gauge. I’m not talking about one of those small face sissy gauges, I’m talking about a real, old-fashioned, gauge like the ones we used to use for testing mechanical fuel pumps with. Connect the gauge to an intake manifold vacuum test reasonably close to the source (long test hoses can dampen the vacuum pulse and make testing less conclusive). To check for a possible exhaust restriction, snap the throttle to wide open and release while watching the vacuum gauge. It should drop to near zero and then return quickly to normal idle readings. If you see the gauge returning slowly, it’s time to take a closer look at the exhaust system.

Another method that makes use of the manifold vacuum gauge is to hold a steady cruise rpm of about 2,500 rpm. Look for your gauge to maintain a steady reading equal to or slightly higher than idle. A reading that progressively drops while holding a constant throttle opening is another indication of a restricted exhaust.

Do you own a Digital Storage Oscilloscope (DSO)? Improper cam timing can cause restrictions to flow by changing when the valves open and close. A quick check for cam timing issues can be performed using starter current draw and an ignition reference. Disable the fuel system to prevent the engine from starting and place your high amp clamp around either battery cable. Select a second channel to capture an ignition event for you reference. Set the voltage divisions appropriately and the time divisions to a half a second per division. Then crank the engine over and grab that waveform.

The peak on each starter current hump is roughly equivalent to Top Dead Center (TDC) of each cylinder’s compression stroke. And when does ignition occur? Just before that, right? And that’s where you should see your ignition trace intersect the current pattern, just before the peak. If the timing is off due to cam timing, the ignition pattern will occur well before (or well after) its normal position. Keep in mind, though, that this is not foolproof. It’s possible to have only one cam in a multiple cam engine to be out of whack. (You can see that in the in-cylinder transducer test I told you about earlier.)

Now We Have To Take Stuff Off Further isolation of a restriction is going to require some work from here on. You can start by removing the upstream oxygen sensors (pre-cat) and installing a backpressure test gauge in its place. Borescopes are increasingly popular tools for actually taking a look at the cat’s condition, and can also be used in the cylinder to see if the valves are, indeed, opening as they should. When you know that you’re facing a VE problem, and your diagnostic tool arsenal is limited, start by verifying and eliminating the obvious first before moving on to the harder stuff. Just remember to think “air flow” and don’t focus on “restriction.”

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